1. Field of the Invention
This invention relates to a luminescent apparatus using a thin film made of a luminescent material, and also to an electric appliance using the luminescent apparatus as a display. An organic EL display and an organic light-emitting diode (OLED) are included in the luminescent apparatus according to the present invention.
The luminescent materials which can be used for the present invention include all luminescent materials that emit light (phosphorescence and/or fluorescence) via singlet excitation or triplet excitation or both thereof.
2. Description of the Related Art
In recent years, the development of a luminescent element (hereinafter referred to as EL element) using a thin film (hereinafter referred to as EL film) made of a luminescent material capable of obtaining EL (electroluminescence) has been forwarded. A luminescent apparatus (hereinafter referred to as EL luminescent apparatus) has an EL element having a structure in which an EL film is held between an anode and a cathode. This apparatus is adapted to obtain luminescence by applying a voltage between the positive and cathode. Especially, an organic film used as an EL film is called an organic EL film.
A metal (typically, a metal of Group I or II on the periodic table) having a small work function is used as a cathode in many cases, while a conductive film (hereinafter referred to as a transparent conductive film) transparent with respect to visible light is used as an anode in many cases. Owing to such a structure, the luminescence obtained passes through the anode, and is visually recognized.
Recently, the development of an active matrix type EL luminescent apparatus adapted to control the luminescence of an EL element provided in each image element by using a TFT (thin film transistor) has been forwarded, and a prototype thereof has come to be made public. The constructions of active matrix type EL luminescent apparatuses are shown in FIGS. 9A and 9B.
Referring to FIG. 9A, a TFT 902 is formed on a substrate 901, and an anode 903 is connected to the TFT 902. An organic EL film 904 and a cathode 905 are formed on the anode 903, and an EL element 906 including the anode 903, organic EL film 904 and cathode 905 is thereby formed.
In this luminescent apparatus, the luminescence generated in the organic EL film 904 passes through the anode 903, and is emitted in the direction of an arrow in the drawing. Therefore, the TFT 902 becomes a luminescence screening object from an observer's viewpoint, and causes an effective emission region (region in which an observer can make observation of luminescence) to be narrowed. In order to obtain a bright image when the effective emission region is narrow, it is necessary to increase an emission brightness but increasing the emission brightness results in an early deterioration of the organic EL film.
Under these circumstances, an active matrix type EL luminescent apparatus of a structure shown in FIG. 9B has been proposed. Referring to FIG. 9B, a TFT 902 is formed on a substrate 901, and a cathode 907 is connected to the TFT 902. An organic EL film 908 and an anode 909 are formed on the cathode 907, and an EL element 910 including the cathode 907, organic EL film 908 and the anode 909 are thereby formed. That is, this EL element 910 constitutes a structure directed contrariwise with respect to the EL element 906 shown in FIG. 9A.
In the luminescent apparatus of FIG. 9B, the luminescence generated theoretically in the EL film 908 passes through the anode 909, and is emitted in the direction of an arrow in the drawing. Accordingly, the TFT 901 enables the whole region, which is provided in a position which cannot be seen by an observer, and which has the electrode 907 thereon, to be used as an effective emission region.
However, the structure shown in FIG. 9B has potentially a problem that the structure is incapable of applying a uniform voltage to the anode 909. It is known that a resistance value of a transparent conductive film used generally as an anode is high as compared with that of a metallic film and can be reduced by thermally treating the transparent film. However, since the organic EL film has a low thermal resistance, a thermal treatment of over 150° C. cannot be conducted after the organic EL film has been formed.
Therefore, when an anode (transparent conductive film) is laminated on an organic EL film, a thermal treatment cannot be conducted, so that it is difficult to form an anode of a low resistance value. That is, there is a possibility that a level of a voltage applied to the anode differs at an end portion and a central portion thereof. There is a fear that this problem causes a decrease in the quality of an image.
As mentioned above, in a luminescent apparatus including a structure using a transparent conductive film formed after the formation of an organic EL film, it is difficult to reduce the resistance of the transparent conductive film.